1. Field of the Invention
This invention relates to a process for preparing urethanes by reacting a solution of a nitrogen-containing organic compound and a hydroxyl-containing organic compound with carbon monoxide in the presence of a halide-free ruthenium catalyst. In the process of this invention, the rate of conversion and selectivity to urethane is increased by providing a primary amine in the reaction solution.
2. Description of the Art
Isocyanates such as toluene di isocyanate (TDI) and 4,4'-diisocyanato diphenyl methane (MDI) are used commercially in the preparation of urethane polymers. The present commercial technology for the preparation of these isocyanates utilizes phosgene, which is costly, toxic, corrosive, and difficult to handle. It is thus understandable that a great deal of recent research has been directed toward different methods for preparing isocyanates, especially TDI and MDI.
Various patents have disclosed methods for carbonylating nitrogen-containing organic compounds, e.g. nitro compounds amines, azo-and azoxy compounds to either isocyanates or urethanes in the presence of a platinum group metal-containing catalyst; usually a palladium or rhodium-containing catalyst, and most often a palladium halide-containing catalyst. (The urethanes can be decomposed to yield the corresponding isocyanates.) Generally, a cocatalyst (promoter) or a coreactant has been utilized in combination with the aforementioned platinum group metal-containing catalysts; Lewis acids, Lewis bases, oxidizing agents, reducing agents, etc. have been used as cocatalysts or coreactants in the platinum group metal-catalyzed carbonylation of nitrogen-containing organic compounds. It is important to note that the vast majority of the research on the carbonylation of nitrogen-containing organic compounds has been directed to catalysis by rhodium or palladium-containing catalysts; especially palladium halide-containing catalysts. Therefore the cocatalysts or coreactants, that have been disclosed, have a demonstrable effect on the activity and selectivity of a palladium-containing catalyst; but the effect of such cocatalysts on the activity or selectivity of other platinum group metals or compounds is speculative.
Due to the complex nature of catalysis, it is often difficult to predict the effect of a known cocatalyst or coreactant on a catalyst having a different metal as the catalytically active moiety. Therefore, although U.S. Pat. No. 4,178,455 discloses that the reaction rate and yield (in a platinum group metal catalyzed process for preparing an aromatic urethane) is increased by a promoter consisting of a Lewis acid (e.g. metal halides, and especially iron chlorides); an organic primary amino compound, a urea compound, a biuret compound, an allophanate compound, or a mixture thereof; it is not obvious that such promoter is effective either in the absence of the Lewis acid or with a platinum group metal or platinum group metal compound other than palladium or palladium chloride. As a result, the effect of the promoter, disclosed in U.S. Pat. No. 4,178,455, on other catalyst metals can not be predicted with a reasonable degree of certainty.
It is known that palladium must be in the Pd.sup.+2 oxidation state to catalyze the production of urethanes from nitro-aromatics; however, during the reaction Pd.sup.+2 is converted to the inactive Pd.sup.o oxidation state. The nitroaromatic can only slowly reoxidize the inactive Pd.sup.o to the active Pd.sup.+2 oxidation state; therefore, an iron compound, or other similar Lewis acids are used as promoters. While not wishing to be bound by theory, it is believed that the palladium-catalyzed carbonylation of nitro aromatics is represented by the following equations, (wherein nitrobenzene and ethanol represent the reactants and PdCl.sub.2 and FeCl.sub.2 represent the palladium catalyst and cocatalyst, respectively). EQU C.sub.6 H.sub.5 NO.sub.2 +6FeCl.sub.2 +6HCl.fwdarw.C.sub.6 H.sub.5 NH.sub.2 +2H.sub.2 O+6FeCl.sub.3 ( 1) EQU C.sub.6 H.sub.5 NH.sub.2 +CO+C.sub.2 H.sub.5 OH+PdCl.sub.2 .fwdarw.C.sub.6 H.sub.5 NHCO.sub.2 C.sub.2 H.sub.5 +Pd.sup.o +2HCl (2) EQU 2x[H.sub.2 O+CO+PdCl.sub.2 .fwdarw.CO.sub.2 +Pd.sup.o +2HCl](3) EQU 3x[Pd.sup.o +2FeCl.sub.3 .fwdarw.PdCl.sub.2 +2FeCl.sub.2 ] (4) EQU Net Reaction: C.sub.6 H.sub.5 NO.sub.2 +3CO+C.sub.2 H.sub.5 OH.fwdarw.C.sub.6 H.sub.5 NHCO.sub.2 C.sub.2 H.sub.5 +2CO.sub.2
In this reaction scheme Fe.sup.+2 reduces nitrobenzene to aniline and water, becoming oxidized to Fe.sup.+3 (1). Pd.sup.+2 converts aniline to urethane, becoming reduced to Pd.sup.o (2). The water and carbon monoxide are shifted to carbon dioxide in another process which reduces Pd.sup.+2 to Pd.sup.o (3). Pd.sup.o is reoxidized by Fe.sup.+3, regenerating Pd.sup.+2 and Fe.sup.+2 (4). The role of iron chlorides or similar Lewis Acids in the Pd-catalyzed carbonylation system may thus be defined as the catalysis of reoxidation of Pd.sup.o by nitroaromatic reduction to the amine. In view of the above, it may be understood, why the palladium catalyzed carbonylation of nitroaromatics requires an iron halide or a similar Lewis Acid as a promoter.
In the few references which suggest that ruthenium compounds are suitable catalysts for the carbonylation of nitrogen-containing organic compounds to the corresponding urethanes or isocyanates, the catalyst is either a ruthenium halide, or a halide-containing moiety is combined with the ruthenium compound to provide the active catalyst. For example, in U.S. Pat. Nos. 3,660,458; 4,134,880; and 4,186,269; the ruthenium compound that has demonstrated catalytic activity is ruthenium chloride. In U.S. Pat. Nos. 3,461,149 and 3,979,427 ruthenium-on-alumina is treated with halide-containing compounds, such as ferric chloride or 1,1,2-trichloro - 1,2,2,-trifluoroethane, to provide a heterogeneous catalyst.
Another example of a heterogeneous ruthenium catalyst for the preparation of aromatic isocyanates may be found in U.S. Pat. No. 3,737,445. This patent discloses a gas-phase process for reacting carbon monoxide with an aromatic nitro or nitroso compound to yield an aromatic isocyanate.
Ruthenium compounds have been utilized in the reduction of organic nitro compounds to the corresponding amines with mixtures of hyrogen and carbon monoxide. It was reported in U.S. Pat. No. 3,729,512 that the reduction of the organic nitro compound with carbon monoxide and ethanol, in the absence of H.sub.2, resulted in a mixture of amine and a urethane. The patentee was not concerned with the preparation of a urethane product; therefore, there was no attempt to increase the selectivity above the approximately 22 percent, urethane, that was obtained.